This invention relates to the field of air distribution of the type common to air conditioning systems in buildings. The invention particularly relates to air induction units used in such systems to induce secondary air into a stream of primary air supplied from a source for delivery to a zone of a building.
Air induction units are well known and typically comprise a box having a primary air inlet normally connected to a source of primary air and an outlet for delivering air to a point of use, such as a room or zone of a building. The induction unit is also typically provided with a secondary air inlet which permits secondary air to be induced or drawn into the unit for mixture with the primary air stream. The induction unit thus produces a stream of mixed primary and secondary air which is delivered to a conditioned zone. Control apparatus is also generally provided so that the amount of primary air can be regulated in response to a sensed condition such as temperature. It is also common to control the amount of secondary air induced in response to a sensed condition. This may be done by providing a damper associated with the secondary air inlet, appropriate control means being provided to operate the damper.
Particular situations are encountered in which it is desirable to provide induction units having high level induction capabilities in order to maximize the amount of secondary air which is induced so as to meet system air flow specifications. For example, energy conservation is a critical factor in the design of building air conditioning systems. One way to reduce system energy requirements is to design the system to operate at lower system primary air pressures than that often employed in older systems. In order to meet specified levels of air flow volume into building zones at these lower primary air pressures, induction of secondary air into the primary air stream must be increased.
Another typical situation in which such high level induction units would be desired is the case of an interior zone of a building in a heating mode in which the amount of additional heat required by the zone to adequately control temperature is relatively small. In such cases only a relatively low volume of primary air flow into the zone may be required with the result that this low volume air flow does not provide adequate air circulation within the zone. A solution to the air circulation problem is to provide an induction unit which permits an increase in the volume of air flowing into the zone by inducing secondary air into the primary air stream. By maximizing the induction of secondary air into the primary air stream air circulation within the zone can be improved. A further factor which may become important is that, generally, a single duct system and primary air supply serve both interior and exterior zones of a building. If a sufficiently high level of secondary air induction is not obtained in the interior zones, the specified volume of air discharged into the zone must be obtained by a higher level of primary air flow. Since the heating requirement of the interior zone is relatively low, a lower temperature supply system would be required. It may then be necessary to provide air distribution units with reheat capabilities in the exterior zones. This increases the cost of installation and operation of reheat units.
Several problems become evident when attempts are made to provide adequate air circulation within a zone having a low primary air volume requirement. A major factor is that space limitations associated with typical building air conditioning systems place size constraints on induction units which may be installed to deliver air to building zones. These induction unit size limitations necessarily limit the relative sizes of primary, secondary, and mixed air flow passages within the induction unit. It is apparent from momentum equations that there is a theoretical limit, determined by the geometry of air flow passages, on the amount of secondary air which can be induced at a single point of induction. Consequently, known induction units which induce air into the primary air stream at a single point have an inherent limit on their induction capabilities. Thus, the use of existing air induction units within system space limitations does not provide the air volume induction level required for satisfactory air circulation within the zone in many cases.
From the foregoing it is apparent that in situations in which it is desirable to obtain maximum induction of secondary air for a given primary air supply pressure, or alternatively, to deliver a specified volume of mixed primary and secondary air to the zone at a reduced primary air supply pressure there is a need for induction units having induction capabilities beyond that presently available in existing induction units.